There's also a theory that that's why we usually find small furry things (like cats) cute as well- our ancestors used to have small furry babies, and we still have the tendency for liking that in our genes.

Michael C. LaBarbera is a professor in Organismal Biology & Anatomy, Geophysical Sciences, the Committee on Evolutionary Biology, and the College of the University of Chicago.

Another Steven Spielberg blockbuster, ET (1982), exhibits an entirely different kind of sophistication. (Okay. So maybe ET isn't a "monster" movie, but bear with me--the point is interesting.) The first time the audience gets a good look at ET, there's an audible sigh of "How cute!" that sweeps through the movie theater. You do indeed think ET is cute, but why do you think he's cute? A few years ago, the late Stephen J. Gould wrote an article for Natural History on the evolution of Mickey Mouse. Gould pointed out that, as time progressed, Mickey Mouse got younger, not older. All mammals go through a regular series of shape changes as they grow and mature. The limbs grow longer with respect to the body, the body grows larger with respect to the head, the head grows larger with respect to the eyes, but the lower part of the face (the "snout") grows larger with respect to the rest of the face and head. These are regular and stereotyped changes in proportion during the development of all mammals. Juvenile mammals, then, are immediately recognizable as such; they have large eyes relative to their heads, small snouts, and disproportionately large heads and short limbs. These characteristics are particularly apparent in the young of our own species, and we're hard-wired with a propensity to react to such characteristics with nurturing behaviors. This is why you find puppies and kittens "cute"—their bodies have the proportions that shout "infant" to your hindbrain. And this is why you find squirrels (which are basically tree rats) cute—even as adults, they have disproportionately large eyes and short snouts. During Mickey Mouse's maturation from his first appearance as Steamboat Willy to his glory days with the Mickey Mouse Club, his head and eyes became larger and his snout proportionally smaller; he juvenilized, presumably increasing his marketability. This is ET's secret for tugging your heartstrings. ET has a very flat face and large eyes, his head is large relative to his body, and his limbs are ridiculously small—his body proportions are exaggerated versions of the children's. Like kittens and squirrels, ET appeals to your hindbrain for nurturing and protection, and it's a rare person who can resist the appeal.